In various situations there is a need for communications where wireless communications is not practical but there is some existing infrastructure such as cables used for other purposes. One example relates to underground mining. Underground mining operations are inherently dangerous for many reasons including the possibilities of miners being exposed to: (1) toxic gas, (2) exposure to fire or explosion related overpressure and (3) rock falls. This is particularly true of underground coal mines as the ore is inherently flammable and it outgases methane, a flammable and potentially explosive gas that is odorless and invisible. There is a long and tragic history of mine accidents resulting usually from methane and coal dust explosions [1,2]. These explosions not only harm miners but frequently destroy key mine infrastructure such as communication devices and associated cabling making a timely mine rescue of injured or endangered miners both difficult and potentially very dangerous. Wireless communication over significant distances in coal mines is generally not possible due to the attenuating effects of the earth and the somewhat conductive coal seams [3].
Power distribution cable is, however, present in all modern underground mines and is inherently very rugged and durable. This cable is usually referred to as ‘high line’ cable and consists of all three power phases bundled into a single very thick cable that is typically hung from the walls (“ribs”) or roof of mines. Modern mines may require a number of such cables that have current capacities of 300 Amps or more and may be designed for 12 kV or more of potential difference between electrical phases. Because of the required current capacity these cables consist of multiple strands of usually 4/0 woven aluminum cables that are inherently resistant to being severed as each is about 0.5″ in diameter with another 0.18″ of insulation per strand [4]. Indeed, there is no published or available record of these cables ever being severed in a mine explosion or rock-fall although there is personal recollection of such a cable being pulled out of its socketed connection in a mine transformer (“power center”) [5].
What is needed are methods, apparatus, and systems for communications which may be used in mines or other applications which do not require the addition of any cabling specifically for communications but can rely on existing power or other cables that may be very rugged and durable.